The documentation deals with an example file using an STL which is in itself dimension less. Here I expect, units are copied one to one from blender to the file.
Only for STL does the upload interface require you to set units, in that case millimeters, as assumed for designing in blender to shapeways/stl.

2.5 and above you can set the units to whatever you want...this is in the scene tab. what i do is set it to metric and the scale to 1- then scale the object to whatever i am making it to... but you have to remember to "apply scale" to all of the objects before exporting. then when i am uploading at SW i use stl and set it to meters... works from the get go.

sory for maybe obvious question but after seting up to metric how i can mesure for ex. my box i want to make box in about 5mm of every edge... and i have no idea how to scale the metrics leave it 1.000 or make it 0.001 ?

Leave it in blender units. when you export to STL set as mm. That tells the program you are exporting it to the dimensions of a basic length of a grids edge length. So just create a cube of width 5,length 5, and height 5 in blender units.
When you export as mm, dimensions will be 5mmx5mmx5mm.

Have any questions regarding Blender, and need fast answers, you are always welcome at the IRC Server Freenode, channel #blender. As a bonus, several there have experience in modelling for 3D prints.

if you are going with what i said then you want it at 1.0. to figure out the size of an object in blender, after you have set the proper scale, blender will tell you on the information panel... if you are looking in at the 3d there will be a plus sign in the top right corner of the 3d area... hit that and a panel will appear telling you what you are looking for... including scale factor of the part selected and rotation and translation... there is other things as well but this is what you are looking for.